CLIMATE ASSISTED EROSIONAL UNROOFING IN THE DABIE MOUNTAINS

Mountain building generally associates with intense denudation and rapid exhumation especially for collisional orogenesis and are modulated by both climate and tectonics. However, the climate effect is usually overlooked. The Dabie Mountains lie in the central east China and mark the collisional suture between the North and South China cratons. There outcropped voluminous ultrahigh-high pressure metamorphic rocks. These rocks were exhumed rapidly from mantle to crust depth in the late Triassic-Early Jurassic. But whether there was massive erosional unroofing during the sub-ground exhumations is still unresolved.

To further understand the Triassic-Jurassic denudation in the Dabie Mountains, we need to know what rocks have been removed. The Neoproterozoic igneous and Paleozoic metasedimentary rocks preserved in the northern Dabie orogen provide a stratigraphic framework to reconstruct the denuded sections. The Late Paleozoic Meishan Group is lightly metamorphosed and used to represent the late Paleozoic successions eroded from the Dabie Orogen. The Neoproterozoic igneous rocks of Luzhenguan Complex in the northern Dabie orogen are correlated to those in the northern South China, and the Silurian quartzites of Foziling Group also display a detrital zircon U-Pb age pattern similar as the Silurian quartz sandstones in the northern South China. We therefore assume that the denuded Neoproterozoic-Silurian successions in the Dabie Orogen are comparable to those in the northern South China. According to this composite stratigraphy, we would anticipant sequential provenance changes for the sediments in the foreland basin if erosional unroofing did execute on the Dabie Mountains during the rapid exhumation.

We conduct a careful provenance analysis on the Triassic-Jurassic sediments in a foreland basin to the south of Dabie Mountains. Our data reveal a fast provenance change in the Late Triassic from the Silurian quartzites to the Neoproterozoic basement. It thus indicates a rapid chemical denudation of the carbonate dominated intervals in between under a coal forming humid climate. The volume of carbonates chemically denuded can be estimated to be ~4 × 10⁴ km³. This rapid chemical denudation of such voluminous carbonate rocks might contribute to the contemporaneous decrease in the atmospheric CO₂ concentration_.